Series fans with flow modification element

ABSTRACT

A series fan assembly has a primary fan, a flow modification element to reduce swirl, and a secondary fan, mounted in a series configuration. A connecting sleeve directs the combined output into an enclosure containing components to be cooled, or a heat sink. A sliding drawer is configured within said connecting sleeve to detachably hold said primary fan, said flow modification element, and said secondary cooling fan, allowing for the hot swappable replacement of defective components. A controller is in communication with a power source, said primary fan, said secondary fan, and at least one sensor monitoring the status of each of said primary fan and said secondary fan. Said controller is configured to maintain said combined output above a minimum control level at all times, in the event of the failure of said primary fan or said secondary fan.

PRIORITY

This application claims priority from U.S. 60/520,678 (High performanceSeries Fan Configurations, filed Nov. 18, 2003) and U.S. 60/520,676(Dual Redundant Cooling Fan Sinks and Trays, filed Nov. 18, 2003)

FIELD OF THE INVENTION

This invention relates to a unique series fan configuration intended forcooling electronics. The configuration is modular, extremely compact,fault tolerant, and uses readily available low cost axial fans. Adisplay panel may be configured to alert the user regarding a failedfan, which may then be replaced (or “hot swapped”) without shutting downthe system being cooled.

ACKNOWLEDGEMENT OF PRIOR ART

The need for highly reliable, fault tolerant, and hot swappable coolingfans has increased as the mission critical use of high performanceelectronics becomes more and more prevalent. In many cases a loss ofcooling for more than a brief moment could damage the underlyingelectronic components.

This has driven a tremendous amount of inventive activity in the fieldas evidenced by numerous recent patents including U.S. Pat. No.6,247,898 issued Jun. 19, 2001 to Henderson, et al (assigned to MicronElectronics), U.S. Pat. No. 6,108,203 issued Aug. 22, 2000 to Dittus, etal (assigned to IBM), U.S. Pat. No. 6,101,459 issued Aug. 8, 2000 toTavallaei, et al (assigned to Compaq Computer), U.S. Pat. No. 6,061,237issued May 9, 2000 to Sands, et al (assigned to Dell Computer), U.S.Pat. No. 6,040,987 issued Mar. 21, 2000 to Schmitt, et al (assigned toDell), U.S. Pat. No. 6,031,717 issued Feb. 29, 2000 to Baddour, et al(assigned to Dell Computer), U.S. Pat. No. 6,021,042 issued Feb. 1, 2000to Anderson, et al (assigned to Intel Corporation), U.S. Pat. No.6,005,770 issued Dec. 21, 1999 to Schmitt (assigned to Dell Computer),U.S. Pat. No. 5,572,403 issued Nov. 5, 1996 to Mills, et al (assigned toDell Computer), and U.S. Pat. No. 5,562,410 issued Oct. 8, 1996 toSachs, et al (assigned to EMC Corporation),

Most of these patents, including U.S. Pat. No. 6,108,203 assigned toIBM, U.S. Pat. No. 6,101,459 assigned to Compaq, U.S. Pat. No. 6,061,237assigned to Dell, U.S. Pat. No. 6,031,717 assigned to Dell, U.S. Pat.No. 6,021,042 assigned to Intel, and U.S. Pat. No. 6,005,770 assigned toDell teach redundant fans operating in parallel. Of these, U.S. Pat. No.6,108,203, U.S. Pat. No. 6,061, 237, U.S. Pat. No. 6,031,717, U.S. Pat.No. 6,021,042, and U.S. Pat. No. 6,005,770 all teach various types ofbaffling to prevent the reverse flow of air through the defective fan,and the ensuing loss of cooling air pressure within the cabinet. U.S.Pat. No. 6,101,459 teaches that this reverse flow of air may beprevented by placing a second, back-up, fan in series with each of theparallel fans. However it must be noted that this same patent alsoteaches that the back-up fans remain idle until required. These patentsalso suggest various ways to ease the process of replacing the defectivefan(s). U.S. Pat. No. 6,061, 237 teaches that two parallel fans may beplaced at an angle to save space.

Only two of these patents, U.S. Pat. No. 6,101,459 assigned to Compaqand U.S. Pat. No. 5,572,403 assigned to Dell, suggest a seriesconfiguration for the cooling fans. Of these, U.S. Pat. No. 6,101,459teaches that the second fan in the series is for back-up purposes only,and will remain idle until required as previously noted. U.S. Pat. No.5,572,403 does teach that the series configured fans run simultaneously,in counter-rotating fashion, and further teaches that a plenum bypass beused to reduce impedance and increase airflow in the event of a fanfailure. However this approach requires specialized fans and alsorequires further baffling within the cabinet to accommodate the plenumbypass flow when required.

An additional two of these patents, U.S. Pat. No. 6,040,981 assigned toDell and U.S. Pat. No. 5,562,410 assigned to EMC address the issues ofeasy fan removal and hot swappable fans. U.S. Pat. No. 6,040,981 teachesa removable fan with camming handle that aligns the fan and re-connectspower in a single operation. U.S. Pat. No. 5,562,410 teaches a selfaligning hot-pluggable fan assembly, primarily to complement the faulttolerant characteristic of RAID based disk arrays.

Finally, U.S. Pat. No. 6,247,898 teaches a method of controlling thespeed of a plurality of fans connected in parallel fashion.

SUMMARY OF THE INVENTION

As taught by prior art, a currently accepted solution is to install dualfans (or blowers) in a parallel configuration such that one fan has thecapacity to cool the entire cabinet, at least on a minimal coolingbasis. In this manner, the failure of one fan can be tolerated withoutdamaging the equipment. While this approach works, the parallelinstallation has the following associated problems; (1) mounting twofans side by side requires twice as much cabinet wall space, andincreases the potential for Electro-Magnetic (EM) leakage through thefan opening, (2) the fail over mechanism must contain sufficientbaffling to prevent air from escaping (or entering) through thedefective fan, a complex and bulky approach, (3) further baffling isrequired to ensure that the air stream is directed consistentlyregardless of which fan is operating, and (4) the system may need to beshut down before replacing the defective fan.

There are benefits to mounting the fans in series rather than inparallel—i.e. place one fan behind the other rather than one fan besidethe other. However the problem with this approach has been that two fansin series do not perform well because the airflow produced by theprimary fan contains swirl, and this does not match the ideal inputconditions for the secondary fan. The secondary fan must have asubstantially reduced level of swirl at its input to operateefficiently.

Despite this drawback, the series configuration solves many of theproblems associated with the parallel configuration; (1) a seriesconfiguration takes less cabinet wall space than a parallelconfiguration, and therefore reduces the potential EM leakage, (2) nobaffling is required to prevent air from escaping through the defectivefan—in fact air must flow through the defective fan in order for theseries configuration to work, (3) no further baffling is required toensure that the air is consistently directed since the two fans aremounted on the same or similar axis, and (4) a defective fan may besafely replaced or “hot swapped” without shutting down the system orcomponents being cooled.

Accordingly the present invention discloses a method of reducing theswirl between the two fans by placing a flow modification element, ordiffuser element, between the two fans, so that the above benefits canbe realized. The present invention also discloses several additionalfeatures that will contribute to functionality, ease of use, ease ofmaintenance, and lower cost such as; (1) an integrated filter/flowcontrol element, (2) a user interface panel to show the status of bothfans and the integrated filter element, (3) the ability to replace thefilter element or the defective fan from outside the cabinet while thesystem is running, and (4) a very compact and modular device that can beinstalled between two industry standard fans to create a highperformance series fan configuration. Further, the present inventiondiscloses many applications for high performance series fans such as forthe cooling of components, heat sinks, system cabinets, and enclosures.

It is commonly known that an axial fan works best if it sees laminarflow on the input side. This condition is met with a single fan sincethere is nothing on the input side to generate swirl. However this isnot the case with a series configuration since the output of the primaryfan, as in the case of all axial fans, contains swirl.

The present invention discloses that this problem may be resolved byplacing a diffuser element between the two fans. The result of placing adiffuser element between the two fans is to substantially reduce theswirl produced by the primary fan before the airflow enters thesecondary fan, thereby increasing the efficiency of the secondary fan.

The use of an intermediate diffuser element will not affect the primaryinherent advantages of a series fan configuration—the airflow willalways be in the same direction, even during a fan failure, and nobaffling changes will be required within the cabinet to re-direct theflow during a fan failure. In the event of a primary (or input) fanfailure, the secondary (or output) fan will continue to “pull” airthrough the diffuser element and move it in the same direction. Likewiseair will continue to flow in the same direction if the secondary fanfails, except that the primary fan will “push” rather than “pull” airthrough the diffuser element.

Although the direction of airflow will remain consistent in a series fanconfiguration with a single fan failure, the volume of airflow will bereduced if the remaining fan continues to operate at the same speed.This is an acceptable situation only if the volume of airflow does notfall below the minimum required to dissipate the heat generated by thecomponents being cooled. The present invention teaches that a controlsystem may be configured to sense the fan failure and adjust theremaining fan speed accordingly, in order to ensure that this minimumrequirement is met until the defective fan can be replaced. This type ofcontrol may be easily implemented since (1) many fans today areavailable with fault sensors to indicate an impending failure/totalfailure and (2) fan speed can be easily controlled by controlling theinput parameters such as voltage, in the case of DC fans, or throughpulse width modulation.

The efficiency of the series fan configuration, while in single fanfailure mode, may be increased by allowing the diffuser element to swingor slide out of the air flow, for example by splitting the diffuserelement down the middle and allowing each half to swing out of the flow,or otherwise partially or completely removing the diffuser element fromthe air flow until the defective fan may be replaced. Further, theefficiency of the series fan configuration, while in a single fanfailure mode, may be increased by partially or completely removing thefailed fan from the configuration until such time as it may be replaced.Further, the efficiency of the series fan configuration, while in asingle fan failure mode, may be increased by providing a diffuserelement bypass channel configured to allow the free flow of air past thediffuser element while in failure mode.

Should a fan fail, the present invention teaches that it may be replacedwithout having to shut down the system or components being cooled. Highperformance series fans may be configured as a “sliding drawer” that canbe pulled away from the cabinet without interrupting the airflow. Thedefective fan may be replaced while the drawer is in the “open”position, and then the drawer may be returned to the “closed” positionwithout affecting system operation or necessitating a system shut down.The control system will detect the new fan, and adjust speedsaccordingly.

In some cases it may be possible to enhance the functionality of thediffuser element by configuring it as a combined filter/diffuserelement, to reduce swirl and prevent particles from entering the systembeing cooled, a combined heat exchanger/diffuser element, to reduceswirl while adding or removing heat from the airflow, a combinedElectro-Magnetic (EM) shield/diffuser element, to reduce swirl whilemaintaining the integrity of the EM shield in the fan opening, or otherpossible combinations. In larger applications the diffuser element maybe active rather than passive so that the flow control parameters may beadjusted and optimized while the high performance series fanconfiguration is operating.

Various configurations are possible including a tightly coupled ormodular arrangement, or a loosely coupled or push/pull arrangement. In atightly coupled arrangement a primary fan and a secondary fan may bemounted at opposite ends of an air channel, in a substantially coaxialconfiguration, such that the air channel contains the diffuser element,and directs the airflow from the output of the primary fan, through thediffuser element, and into the secondary fan. In a loosely coupled orpush/pull arrangement a primary fan blows air into an enclosed space anda secondary fan blows air out of the same enclosed space, and thecomponents within the enclosed space act as a type of diffuser elementto remove swirl from the airflow as it moves from the primary to thesecondary fan. In some loosely coupled configurations a diffuser elementmay also be installed on the input side of the secondary fan to furtherreduce the swirl and improve the efficiency of the secondary fan, andbaffling may be added to improve the efficiency of the airflow withinthe enclosed space.

The performance of the secondary fan may be enhanced by increasing theresidual momentum and reducing the swirl component of the airflow at itsinput, as previously described. The primary fan contributes to thisenhanced performance, since it increases the residual momentum of theairflow entering the secondary fan, however it also introduces a swirlcomponent that is counter-productive. An optimized high performanceseries fan configuration retains a maximum level of residual momentumwhile reducing swirl to an ideal level before the airflow enters thesecondary fan.

The total output of a series fan configuration, relative to thetheoretical output of a non-optimized series fan configuration(generally considered to generate two times the static pressure for anygiven CFM output), may be expressed, in simple terms, as follows;Output_(T)=(2×Outputs)+M−S   (1)

-   -   Where Output_(T)=Total output        -   Output_(S)=Output from single fan        -   M=Momentum Factor (at secondary fan)        -   S=Swirl Factor (at secondary fan)

The momentum factor will naturally decay as the distance between theprimary and secondary fans is increased, and as more restrictions, e.g.a diffuser element, are placed in the airflow. From this perspective themost effective series fan configurations will have the least possibledistance between the primary and secondary fans, the closest co-axialalignment between the two fans, and the least number of restrictionsbetween the two fans.

The swirl component will also naturally decay as the distance betweenthe primary and secondary fans is increased, and from this perspectivethe most effective series fan configurations will have the greatestpossible distance between the primary and secondary fans. The presentinvention teaches that this distance may be substantially reduced byinstalling a diffuser element between the primary and secondary fans toforce a more rapid decay of swirl, as previously described. In a looselycoupled series fan configuration the components to be cooled may serveas a type of diffuser element, as in the case of a computer system wherethe primary and secondary fans are located at opposite ends of thecabinet and the air flowing between them must pass over the electroniccomponents. Alternatively the diffuser element may be a purpose builtcomponent placed strategically between the two fans, or in front of thesecondary fan. In either case the flow straightening element(s) willhave both a positive and a negative effect since will it reduce theswirl component while at the same time increasing drag.

Based on this information the model may be re-constructed as follows;Output_(T)=(2×Outputs)+M−S+(S _(R) −D)   (2)

-   -   Where S_(R)=Swirl reduction factor        -   D=Drag introduced by swirl reducing components

Equation (2) may be re-written as follows to separate the behaviour ofthe primary and secondary fans, and associate this behaviour with themost closely aligned correction factor, as observed;Output_(T)=(Output_(SP) −D)+(Output_(SS) +M−(S−S _(R)))   (3)

-   -   Where Output_(SP)=Output of a single primary fan        -   Output_(SS)=Output of a single secondary fan

It is important to note that Output_(SP) and Output_(SS) both representthe output of single fans operating in independent fashion. It followsthat Output_(SP) and Output_(SS) will be the same for a symmetricalseries fan configuration, where the primary and secondary fans haveidentical specifications, and that Output_(SP) and Output_(SS) will bedifferent for an asymmetrical series fan configuration, where theprimary and secondary fans may have different specifications.

Clearly, then, the optimization objectives are to simultaneouslymaximize the momentum of airflow as it enters the secondary fan (M),minimize the swirl component of the airflow as it enters the secondaryfan (S−S_(R)), and minimize the drag introduced by the swirl reducingcomponents (D). In fact the output of the secondary fan may be enhanced,in this manner, to the extent that it exceeds Output_(SS), i.e. itexceeds the output of a single secondary fan operating in independentfashion with input conditions that meet design specifications. Itfollows that the total output of a high performance series fanconfiguration with a diffuser element may exceed the theoretical outputof two single fans as long as the following optimum condition exists;M>((S−S _(R))−D)   (4)

It has been found that an optimal condition may achieved by (1) mountingthe primary and secondary fans coaxially at either end of a sealed airconducting tube or connecting sleeve, adapted with internal featuressuch as longitudinal grooves or octagonal corners to induce naturalswirl decay while maintaining the maximum level of momentum as the airflows between the two fans, and (2) by placing the diffuser element at adistance from the primary fan such that a substantial amount of naturalswirl decay will have occurred before the airflow enters the diffuserelement, as depicted in FIG. 24 (with reference to the followingcomponents and corresponding numbers for FIG. 24 only); Component No.Component No. Primary Fan 200 Secondary Fan 202 Diffuser Element 204Seal 206 Airflow 208 Integrated Stator 210 Acoustic Gap 212

The diffuser element may be further optimized to remove substantiallyall of the remaining swirl while introducing a minimal level ofincremental drag, thereby “straightening” the airflow while maintainingits momentum at the highest possible level as it leaves the diffuserelement, and converting swirl energy to kinetic energy with the highestpossible efficiency. The diffuser element may be placed immediatelybefore or in close proximity to the secondary fan in order to maintainthis momentum as the airflow enters the secondary fan, recognizing thata small gap may be required between the diffuser element and secondaryfan to reduce the acoustical noise produced by the overallconfiguration. The diffuser element and the air conducting tube may becombined and further adapted in various ways to provide furtheroptimization and enhanced performance.

Further optimization may be achieved by controlling the combinedmomentum and swirl at the input to the secondary fan such that themomentum vector(s) drive the secondary fan to achieve greater efficiencyand performance. Such optimization may require a more complex diffuserelement design, optimized for efficient swirl energy to kinetic energyconversion, directional control of the momentum vector(s), reduced drag,and so on.

Further optimization may also be achieved by using a primary fan with anintegrated stator on the output side. In this case Output_(SP) will haveless swirl (due to the straightening effect of the stator) and a lowerflow rate (due to the drag effects of the stator) relative to a similarprimary fan that does not have an integrated stator. These attributescan be used to enhance the performance of, and reduce the overall lengthof, a high performance series fan configuration with diffuser elementsince the requirement for swirl reduction in the area between the twofans will have been reduced by the integrated stator on the primary fan.However the reduced level of drag produced by the shorter air conductingtube between the two fans, and the smaller diffuser element, may beoffset by the Incremental drag produced by the integrated stator on theprimary fan.

A closely coupled high performance series fan with diffuser element, ordual redundant fan module, is ideally suited for the cooling of cabinetsand other enclosures. Further, the excellent single stream performanceunder high static pressures makes it ideal for the impingement coolingof CPUs and other electronic components, as well as the impingementcooling of power heat sinks. The latter configuration may be referred toas a high performance series fan sink.

A loosely coupled or “push/pull” series configuration is depicted inFIG. 25 (with reference to the following components and correspondingnumbers for FIG. 25 and FIG. 26 only); Component No. Component No.Primary Fan 300 Secondary Fan 302 Air Flow In 304 Electronic Components306 System Cabinet 308 Air Flow Out 310

A loosely coupled series configuration may be designed to incorporatesome of these operating parameters, however it will likely deliversub-optimal performance relative to a closely coupled configurationusing similar fans. A loosely coupled series configuration has a muchlarger distance and a much less efficient duct between the primary andsecondary fans, as illustrated below. The result is a substantial lossof momentum before the airflow reaches the secondary fan.

The practice of relying on the electronic and other components to removeswirl may work to some degree, however it would be extremely difficultto lay out the components for the optimization of this function, anddoing so may introduce volumetric inefficiencies in the design. Further,it would be extremely difficult to configure the components such thatsubstantially all of the swirl will have been removed just as theairflow enters the secondary fan. Further, the optimized design, if itcould be achieved, would change with the addition or modification of asingle component within the air space between the two fans.

In contrast, a tightly coupled or modular series fan configurationoperates with an optimized design that remains the same regardless ofcomponent layout within the system cabinet being cooled. While a changein components may affect the static pressure or load conditions, it willnot affect the optimized design of the high performance series fanconfiguration. In other words the performance curve (i.e. staticpressure/flow curve) for the high performance series fan configurationwill remain the same regardless of the change in load curves—it is justthe intersection of these curves (i.e. the operating point) that willchange. The fact that the output of an optimized high performance seriesfan configuration may be plotted as a standard performance curve greatlyeases the thermal design task since the operating point may be readilydetermined in the same way that one would determine the operating pointfor a single fan.

It is possible to combine some of the benefits of a tightly coupledseries configuration with a loosely coupled series configuration byplacing a diffuser element immediately prior to the secondary fan asdepicted in FIG. 26 (with reference to the following components andcorresponding numbers for FIG. 25 and FIG. 26 only); Component No.Component No. Primary Fan 300 Secondary Fan 302 Air Flow In 304Electronic Components 306 System Cabinet 308 Air Flow Out 310 DiffuserElement 312

The installation of a diffuser element at this point in the looselycoupled configuration will serve to remove substantially all of theswirl before the air enters the secondary fan, providing an increase inefficiency as described above.

A further analysis of equation (3) above reveals that the configurationmay be more responsive to an increased level of power applied to thesecondary fan relative to the primary fan. This is due to the fact thatthe impact of any incremental power applied to the secondary fan isenhanced beyond what one would normally expect from a single independentfan because of the increased momentum of the air entering the secondaryfan. When operating independently, the momentum of the air flowing intoand out of the secondary fan is completely generated by the secondaryfan. When operating in a series configuration, however, the air flowingthrough the secondary fan has a residual momentum that has already beengenerated by the primary fan. This increases the efficiency of thesecondary fan beyond that of an independent fan.

A further observed effect is that the primary fan is more sensitive(than the secondary fan) to the drag introduced by the diffuser elementas noted in equation (3). This also indicates that the seriesconfiguration may be more responsive to increased power applied to thesecondary fan rather than the primary fan.

It is therefore possible to take advantage of these effects, andincrease the efficiency of the overall series fan configuration, byre-balancing the distribution of power such that more power is appliedto the secondary fan than the primary fan. The result will be anincreased output relative to an equal distribution of the same totalpower between the two fans. This principle may be applied to tightlycoupled or loosely coupled series fan configurations. In practice it maybe implemented by supplying a higher voltage to the secondary fan thanthe primary fan, or by utilizing a higher performance secondary fan andapplying the same voltage to both fans, or through some other means.

It is important to note that although the preceding discussion has beenlimited to high performance series fan configurations with two fans, theprinciples taught herein may also be applied to configurations of threeor more fans in various series combinations. As an example, a tightlycoupled serial fan module may replace the primary fan in a looselycoupled configuration, resulting in a three (3) fan configuration withenhanced performance.

Further, multiple high performance series fan modules may be installedin parallel for greater airflow capacity and/or to provide multiplefault tolerant airflows. It has been previously noted that parallelsingle fan installations are not inherently fault tolerant since thefailed fan presents an air leak that quickly disperses the pressure andairflow produced by the remaining fan(s). In contrast, a parallelinstallation of two or more high performance series fan modules is faulttolerant because each one of the series fan modules is inherently faulttolerant. The module that contains the failed fan will still continue toproduce airflow and pressure, thereby preventing the leakage of air thatis normally associated with a parallel fan installation. As an addedbenefit, the failed fan may be replaced on a scheduled rather than anurgent basis.

Parallel high performance series fan modules are ideal for manyapplications including system cabinet cooling and rack mount enclosurecooling. The former is particularly well suited for very low profile 1 Uand 2 U (approximately 44 mm and 88 mm in height, respectively) serverformats where the installation of larger diameter fans is impossible andperformance and fault tolerance are essential. The latter configurationmay be used to replace the parallel single fans commonly installed on afan tray to form a high performance series fan tray.

A high performance series fan configuration operates in fault mode whenone fan fails, and the remaining fan continues to create airflow. Acontroller may be configured to recognize and respond to this situationby increasing the power supplied to the remaining fan, therebyincreasing the output during failure mode. In some applications thatdemand improved fault mode performance a unique offset seriesconfiguration provides a supplementary air inlet or air outlet that maybe opened in the event of a fan failure to improve the efficiency of theremaining fan, while maintaining a consistent direction and rate of flow

Finally, the principles taught herein may be applied to larger fans andpropellers to develop high performance fault tolerant automotive fans,e.g. for cooling and turbo-charging, innovative consumer products, suchas vertical pole fans to de-stratify the air within a room, highperformance fault tolerant industrial fans, e.g. for large air movingsystems, propulsion systems, where the safety associated with a faulttolerant configuration cannot be underestimated, and other applicationsthat may become obvious when the principles are understood. Further, theprinciples taught herein may also be applied to other gasses and fluids,e.g. for the development of pumps and marine propulsion systems, andother applications that may becomes obvious when the principles areunderstood.

Embodiments

Embodiments of the invention are described by way of example withreference to the drawings in which:

FIG. 1 illustrates an inefficient series fan configuration,

FIG. 2 illustrates an efficient series fan configuration with diffuserelements,

FIG. 3 provides an overview of a high performance series fanconfiguration,

FIG. 4 provides a side view of a high performance series fanconfiguration,

FIG. 5 provides a side view of a high performance series fan in normaloperation,

FIG. 6 provides a front view of a high performance series fan with acontrol panel,

FIG. 7 illustrates how a high performance series fan drawer may bewithdrawn from a cabinet,

FIG. 8 details the replacement of one of the series fans,

FIG. 9 shows how two high performance series fan modules may be mountedin parallel,

FIG. 10 shows a high performance series fan module with a supplementaryair inlet and outlet,

FIG. 11 provides a connection diagram for a high performance series fancontroller,

FIG. 12 illustrates a control algorithm for a high performance seriesfan controller in flow chart format,

FIG. 13 provides a perspective view of a high performance series fansink,

FIG. 14 provides a section view of a high performance series fan sink,

FIG. 15 illustrates a high performance series fan sink with the primaryfan being replaced,

FIG. 16 illustrates a high performance series fan sink with thesecondary fan being replaced,

FIG. 17 provides a perspective view of a high performance series fantray,

FIG. 18 provides a second perspective view of a high performance seriesfan tray showing further details of one of the high performance seriesfan modules,

FIG. 19 illustrates a high performance series fan tray with the primaryfan being replaced,

FIG. 20 illustrates a high performance series fan tray with thesecondary fan being replaced,

FIG. 21 illustrates a high performance series fan tray controlleroperating in a fan failure mode,

FIG. 22 illustrates a method for monitoring airflow through a highperformance series fan module, and:

FIG. 23 provides a perspective view of an alternatively configured highperformance series fan tray.

FIG. 1 illustrates an inefficient series fan configuration with threeindependent axial cooling fans mounted such that the output from one fanbecomes the input to the next fan in the series. In this case the outputfrom primary fan 8 becomes the input to secondary fan 16, and in likemanner the output from secondary fan 16 becomes the input to tertiaryfan 17. Basic series fan configurations may be comprised of two or moreaxial fans configured in this manner.

An axial fan works best if it sees a substantially laminar flow, i.e. aflow with no or a controlled level of swirl, on the input side. Thiscondition is met with a single fan since there is nothing on the inputside to generate swirl. However this is not the case with a basic seriesconfiguration since the outputs of the primary fan 8 and secondary fan16 (as with all axial fans) contain swirl as depicted by airflow withswirl 10 and second airflow with swirl 11. Therefore a basic seriesconfiguration is inefficient because the secondary, tertiary, and allsubsequent fans will have a substantial swirl component in the inputairflow.

In contrast, FIG. 2 illustrates an efficient series fan configurationwith diffuser element 14 and second diffuser element 15 inserted betweenprimary fan 8 and secondary fan 16, and secondary fan 16 and tertiaryfan 17, respectively.

The result of inserting diffuser element 14 between primary fan 8 andsecondary fan 16 is to convert the input seen by secondary fan 16 fromairflow with swirl 10 to reduced swirl airflow 12, thereby increasingthe efficiency of secondary fan 16 to a level approaching that ofprimary fan 8. Likewise, second diffuser element 15 will convert theinput seen by tertiary fan 17 from second airflow with swirl 11 tosecond reduced swirl airflow 13, thereby improving the efficiency oftertiary fan 17.

Diffuser element 14 and second diffuser element 15 may be comprised, forexample, of filter material or a number of vanes or tubes mounted in thepath of the air and configured to reduce swirl and direct the airflowinto downstream fan, as illustrated by alternative second diffuserelement 15 a. Further, the vanes or tubes may be configured to leave acertain level of residual swirl in the airflow in order to (1) flow moreeasily past the stationary fan blades of a the downstream fan and/or (2)create a set of input conditions that would allow the downstream fan tooperate more efficiently, at above design conditions, rotating fasterthan normal for a given input power level. In certain applications itmay be beneficial to combine diffuser element 14 and second diffuserelement 15 with other functions such as a heat exchanger to add orremove heat from the airflow, or an Electro-Magnetic (EM) shield tosubstantially prevent the passage of EM waves through the fan opening.While the number of different diffuser element designs and their relatedefficiencies and functionalities is vast, the principle of reducingswirl to improve the efficiency of the secondary or downstream fanremains the same.

While diffuser element 14 and second diffuser element 15 may beprimarily designed to reduce swirl, they will also add an impedance tothe airflow that will add to the system head and reduce the efficiencyof the system. This becomes a trade-off that must be balanced againstthe positive effects of installing a diffuser element between two fansin series. In general, however, the overall effect of installing adiffuser element is positive since the impact of the reduced swirl faroutweighs the incremental system head. In some applications the pressuredrop across the diffuser element may be monitored and used to measurethe airflow through the diffuser element.

FIG. 2 also illustrates the impact of a fan failure. If primary fan 8fails, then secondary fan 16 and tertiary fan 17 will continue to drawair through the assembly and “push” it in the same direction, i.e.combined airflow 22 will continue to flow in the same direction, and noexternal baffling changes will be required. A similar result will occurif secondary fan 16 or tertiary fan 17 fails. This ability to continueto provide airflow in the same direction despite the loss of a fan isthe primary inherent advantage of a series fan configuration.

In the event of a primary fan 8 failure, the fan blades may continue torotate or they may remain fixed or “locked”—depending on the nature ofthe failure. However, in the case of primary fan with variable pitchblades 8 a, primary fan blade 9 will remain in an oblique positionduring normal operation (i.e. while rotating in the direction defined byarrow 7) and then return to coaxial position 9 a in the event of afailure. Since coaxial position 9 a aligns the fan blade with theairflow, it will present a far lower input impedance as seen bysecondary fan 16, therefore contributing to increased efficiency duringa primary fan with variable pitch blades 8 a failure relative to anprimary fan 8 (i.e. fixed fan blade) failure. It follows that asecondary fan 16 with similar variable pitch blades would alsocontribute to greater efficiency during the failure mode as it wouldpresent a lower output impedance as seen by primary fan 8.

Although the direction of airflow will remain consistent in a series fanconfiguration with a single fan failure, the volume of airflow will bereduced if the remaining fan(s) continue to operate at the same speed.This is an acceptable situation only if the volume of airflow does notfall below the minimum required to dissipate the heat generated in thecabinet or by the components being cooled. In practice a control systemmay be required to sense the fan failure and adjust the remaining fanspeed accordingly, in order to ensure that this minimum airflowrequirement is met until the defective fan can be replaced. This type ofcontrol can be easily implemented since (1) many fans today areavailable with fault sensors to indicate an impending failure/totalfailure and (2) fan speed can be easily controlled by varying the inputvoltage, at least for DC fans, or by using some other type of fan speedcontroller.

During normal operation, primary fan 8, secondary fan 16, and tertiaryfan 17 may all be operating at less than full rpm to produce therequired combined airflow 22. The lower rpm will reduce the noiseproduced by each fan and also extend the life of each fan. Should thecontroller sense an impending or actual failure in one of these fans,then the. The user may then be alerted to replace the defective fan on ascheduled rather than an urgent basis. Similarly, if the airflow isimpeded by a clogged air filter or some other obstacle, then the powerapplied to the fans may be increased to the point where combined airflow22 remains the same.

A series configuration of “n+1” fans configured with intermediatediffuser elements, as described above, will be tolerant to the failureof one fan where “n” is the total number of fans whose combined flow isrequired to meet the cooling requirements of the system or component(s)being cooled. FIG. 2 illustrates an example where “n”=2, and “n+1”=3fans in total. Actual configurations may include 2, 3 or more fansdepending on cooling requirements. The remainder of this document willdeal with high performance series fans with diffuser elements configuredwith two fans for reasons of simplicity, however it should always benoted that additional fans may be added to these representative seriesconfigurations. Further, it should be noted that multiple fans could beadded to provide increased performance while preserving an n+1redundancy and providing a fault tolerant configuration.

It is also possible that multiple series fans with diffuser or flowmodification elements may be installed in parallel to meet demandingcooling requirements. In this case, there is no need for the movablebaffles normally associated with parallel configurations since eachindependent high performance series fans with diffuser element assemblyis fault tolerant and will not allow the back flow or “leakage” of airin the event of a fan failure. These configurations may be used to meetvery high airflow requirements, to produce independently directedairflow streams, or where space considerations limit the number of fansthat may be mounted in a series.

Series fans with flow modification element, or high performance seriesfans, may be configured to allow a defective fan to be replaced withouthaving to shut down the system or components being cooled—commonlyreferred to as “hot swapping” the fans. This is made possible by thefact that high performance series fans 1 may be configured to fit in asliding “drawer” that can be pulled away from the cabinet withoutinterrupting the airflow, as illustrated in FIG. 3. In this casesecondary fan 16 is being replaced while sliding drawer 2 is in the “outposition. Sliding drawer 2 may then be returned to the “in” positionwithout affecting system operation or necessitating a system shut down.A control system may be configured to detect the fan failure, alert theuser, detect the presence of a new and fully functional secondary fan16, adjust the power applied to both primary fan 8 and secondary fan 16to maintain a controlled airflow throughout the process, and then resetthe lights on control panel 30 to reflect normal operation. Note thatdiffuser element 14 could also be replaced while the sliding drawer 2 isin the “out” position, again without affecting system operation. Fingerguard 6 has been added to the configuration for safety reasons.

FIG. 4 provides further detail in a side view of high performance seriesfans 1 mounted in sliding drawer 2. Primary fan 8 and secondary fan 16are mounted co-axially in sliding drawer 2 such that the air flowingfrom primary fan 8 flows through diffuser element 14 and directly intosecondary fan 16. Sliding drawer 2 slides into and out of internalsleeve 3 as depicted by drawer movement arrow 18. Sliding drawer 2requires a minimum opening in cabinet 4, taking less cabinet wall spacethan a parallel configuration and making it easier to maintain theintegrity of an EM shield. In certain applications diffuser element 14may be configured as an integral part of the EM shield.

Internal sleeve 3 has at least five distinct functions; (1) to provide ameans to mount sliding drawer 2, and therefore high performance seriesfans with diffuser element 1, on cabinet 4, (2) to provide a means toallow sliding drawer 2 to slide “in” or “out”, (3) to support slidingdrawer 2 whilst in the “in” or “out” position, (4) to provide bafflingsuch that combined airflow 22 only exits the assembly through the openend of internal sleeve 3, and (5) to provide, in combination withsliding drawer 2, a contained channel for the air flowing through highperformance series fans 1.

The latter function is particularly important since the length andgeometry of the contained air channel between primary fan 8 and diffuserelement 14 may be configured to provide a pre-determined level ofnatural decay of swirl in the airflow before it enters diffuser element14. This natural decay of swirl may be enhanced by providing multiplecorners within this portion of the contained air channel, for example byconfiguring the air channel with a square or hexagonal cross section. Incertain applications, in particular those using a primary fan 8 havingstator blades, the this portion of the contained air channel may beshortened while providing the same overall effect since some of theswirl will have already been removed by the stator blades.

Similarly the length and geometry of the contained air channel betweenprimary fan 8 and diffuser element 14, and diffuser element 14 andsecondary fan 16, may be configured to reduce the acoustical noiseproduced by high performance series fans 1. As an example, a shortcontained air channel with smooth walls between diffuser element 14 andsecondary fan 16 may be configured to reduce acoustical noise, eventhough it may not necessarily be required to further reduce swirl inthis region.

Flange 21 may be used to secure internal sleeve 3 to cabinet 4 withmachine screws, or through some other suitable means. Latch 19 may beused to hold and seal tab 20 against flange 21, i.e. to hold slidingdrawer 2 in the “in” position, until released. Back lip 5 extendsoutward from the normal geometry of sliding drawer 2 to prevent theaccidental removal of sliding drawer 2 by coming to rest against anextended portion of flange 21, when sliding drawer 2 is in the full“out” position. A means may be provided to completely remove slidingdrawer 2 from internal sleeve 3, when and if required.

In some applications diffuser element 14 may be configured as adiffuser, to reduce swirl in the airflow leaving primary fan 8, and as afilter, to substantially remove unwanted particulate from the airflow.In these cases diffuser element 14 should be selected to optimize bothfunctions, in combination with the length and geometry of the containedair channel between primary fan 8 and diffuser element 14, as describedabove, while introducing a minimal incremental system head.

Alternatively, an air filter optimized for removing particulates may bemounted between finger guard 6 and primary fan 8, leaving the diffuserelement 14 to be fully optimized for the reduction of swirl. In theseconfigurations diffuser element 14 may be a screen, a laminar flowelement consisting of a number of round, square, hexagonal, oralternatively shaped tubes mounted co-axially with the fans, a series offlow directing vanes, or some combination thereof. Further, diffuserelement 14 may be configured with an air funnel at the entry point toeach tube, and with the funnel openings directed/skewed towards thesource of the air as it comes off the blades of primary fan 8.Regardless of configuration, the flow related objective of diffuserelement 14 is, in combination with the length and geometry of thecontained air channel between primary fan 8 and diffuser element 14, toreduce swirl in the airflow leaving primary fan 8, and before it enterssecondary fan 16, while introducing a minimum amount of incremental backpressure, thereby contributing to the overall efficiency of the highperformance series fans 1.

Primary fan 8 and secondary fan 16 may rotate in the same or differentdirections. This aspect of the configuration will be somewhat dependenton the cost, performance, and acoustical objectives associated with agiven application, as a pair of standard fans that rotate in the samedirection may be less expensive than a pair of counter-rotating fans, ora counter-rotating fan module. Also, any efficiency gained by havingcounter-rotating fans should be weighed against the service cost ofstocking two types of spares.

FIG. 5 shows high performance series fans 1 in operation. In this casesliding drawer 2 has been moved “in” such that finger guard 6 is flushwith the outside of cabinet 4. Sliding drawer 2 slides within theinternal sleeve with sliding interfaces at flange 21 and back lip 5.Alternatively, sliding drawer 2 may be configured to slide on rails orsome other suitable means.

Sliding drawer 2 is prevented from moving farther into cabinet 4 by tab20 (top and bottom) when it interfaces with the outer edge of flange21.Sliding drawer 2 is then held in place by latch 19. In some cases anaesthetic cover may be configured to snap onto the outside of slidingdrawer 2, once in place, to improve the appearance of the coolingmodule. Further, the aesthetic cover would provide visual access to thecontrol panel so that the operation of high performance series fans 1could still be easily monitored.

As in FIG. 4, cooling air flows efficiently through primary fan 8,diffuser element 14, and secondary fan 16 to provide combined airflow22. It is important to note that the direction of combined airflow 22remains consistent whether one or both of primary fan 8 and secondaryfan 16 is/are operational. This precludes the requirement for anyincremental baffling to ensure that the direction of combined airflow 22remains consistent in the event of a fan failure.

In the event of a primary fan 8 failure, combined airflow 22 willcontinue to flow through primary fan 8 and into secondary fan 16—i.e.the airflow will not escape through primary fan 8. Likewise, in theevent of an secondary fan 16 failure, combined airflow 22 will continueto flow through secondary fan 16 and into cabinet 4—i.e. the airflowwill not escape through secondary fan 16. This precludes the requirementfor specialize baffling to prevent combined airflow 22 from escapingthrough the defective fan.

The last two paragraphs highlight a very important characteristic of theseries fan configuration—no baffling is required to accommodate a failedfan scenario. This contrasts sharply with the parallel fan configurationwhere substantial baffling is required to prevent the loss of airthrough the defective fan and to keep the direction of airflowconsistent in the event of a fan failure. As a result, high performanceseries fans are very compact, and they may be configured as astand-alone cooling module that does not requires any further baffling.

Primary fan 8 may need to be rated at a higher capacity than secondaryfan 16 to compensate for the added backpressure introduced by diffuserelement 14 and secondary fan 16, if and when secondary fan 16 isdefective and/or stationary. Conversely stated, secondary fan 16 may berated at a lower capacity than primary fan 8 because it will not “see”the same incremental causes of backpressure. In practice both fans maybe of the same rating, but should they be so configured that the ratingsmatch the higher rating required by primary fan 8. This will ensure thatcombined airflow 22 will always exceed the minimum required regardlessof whether one or both fans is/are operational.

During normal operation primary fan 8 and secondary fan 16 may run atless than full rpm as long as combined airflow 22 meets the coolingrequirements for the application at hand. Further, the total powerapplied to the system may be re-balanced asymmetrically, with more powerbeing applied to the secondary fan in order to take advantage of thefact that secondary fan 16 runs more efficiently than primary fan 8,therefore improving the overall efficiency of the system. Theconfiguration will be very responsive to a fan failure since theremaining fan is already running, albeit at a lower rpm, and it is muchfaster to ramp up from partial to full rpm than it is to go from stoppedto full rpm.

It can be deduced from FIG. 5 that the size of the opening in cabinet 4will be only slightly larger than the size of primary fan 8. In aparallel configuration the opening would be approximately twice thissize since the two fans would be mounted side-by-side. Further thevolume of space required in cabinet 4 will be much smaller than aparallel configuration since no extra internal baffling will berequired. This 2:1 reduction in the size of the opening combined withthe much smaller internal volume requirement represents a major benefitof the series configuration from a system designer's perspective.

In simple configurations, high performance series fans 1 may beimplemented without a controller by using two fans, each of which iscapable of providing the full combined airflow 22 required for theapplication at hand. Under normal operating conditions combined airflow22 will actually exceed the minimum requirement, keeping the load coolerthan necessary. A fan failure can be tolerated since the remaining fanwill already be running, and is capable of carrying the load. Asdescribed above, no further baffling is required since the fans are inseries. A simple indicator light will flag the operator to replace thedefective fan.

In other configurations, where power consumption, precise cooling,and/or acoustic management are important requirements, a controller maybe used to provide a controlled airflow during normal operation and inthe event of a fan failure. The controller may be installed behindcontrol panel 30, as shown in FIG. 6. This drawing also illustrates thefull extent of tab 20 as seen around the perimeter of the unit, and thefront face of finger guard 6. Control panel 30 contains indicator lights32 to alert the user regarding the operation of primary fan 8, secondaryfan 16, and diffuser element 14 (reference FIG. 5). The controller mayalso be adapted to communicate with other systems for remote monitoringand control.

An aesthetic cover may be affixed over the entire front face of highperformance series fan 1, providing that airflow is not impeded to thedegree that it will affect cooling performance. In most cases indicatorlights 32 will need to be visible through the aesthetic cover so thatthe operator can respond to a fan problem, however this may not be anabsolute requirement in situations where the operator may be initiallyalerted through some other means, for example through software and aremote monitor. In the latter case the operator, once alerted to theproblem, could remove the aesthetic cover and visually inspect indicatorlights 32 to determine which fan is defective.

Fans are readily available with sensors for failure, or degradation inperformance that might indicate imminent failure. This information maybe used to inform the controller to increase the speed of the other fanin order to continue to provide the required airflow. The controller canalso use the same information to illuminate the appropriate indicatorlights 32, alerting the operator to take action. Indicator lights 32 maybe activated in several different modes, e.g. steady, flashing, redyellow or green, to communicate certain information and the level ofseverity of the problem to the user.

Under normal operation each fan may be running at less than maximum rpmto extend life, reduce noise, and to allow for an immediate increase inspeed should the other fan fail. It is possible that one fan may be leftidle (i.e. not running) during normal operation, however in practice itmay be better to leave both fans running to some extent in order to (1)continually ensure that they are both operational (2) minimize any “rampup” time in the event of a failure and (3) reduce any unnecessary staticloads or sources of backpressure during normal operation.

FIG. 7 illustrates how high performance series fans 1 may be withdrawnfrom cabinet 4 to allow for the inspection and/or replacement of afaulty component. Note that finger guard 6 has been removed in thisdiagram for illustrative purposes only, and that this would not normallybe the case when servicing the unit.

FIG. 8 provides a top view of high performance series fans 1, andillustrates the method of replacing a defective fan without shuttingdown the system, commonly referred to as “hot swapping” the fans. Inthis scenario secondary fan 16 is defective, and this information wouldhave been conveyed to the user through indicator lights 32.

The first step in replacing defective secondary fan 16 is to pull outsliding drawer 2 until it is fully extended, as depicted by drawerextension arrow 42. At this point back lip 5 will rest against theinternal edge of flange 21 to prevent further forward movement ofsliding drawer 2. Internal indicator lights 33 may be used as asecondary check to ensure that the correct (faulty) fan is beingremoved.

Once sliding drawer 2 is in the fully extended position, secondary fan16 may be removed by sliding it sideways, to the right, anddisconnecting internal power and control cable 44 from internal powerand control receptacle 46. FIG. 8 shows secondary fan 16 partiallyremoved with approximately 30% of its width already beyond the rightside of sliding drawer 2. Note that secondary fan 16 is completelyoutside of and can slide clear of cabinet 4. It can be seen thatdiffuser element 14 and primary fan 8 could be similarly removed withoutinterfering with cabinet 4.

Primary fan 8 remains running as secondary fan 16 is being removed andreplaced, and may be running at a higher RPM, as determined bycontroller 40, so that combined airflow 22 remains at or above theminimum airflow required to cool the components contained within cabinet4. Note that the direction of combined airflow 22 will not change, as itremains contained and directed by internal sleeve 3, precluding the needfor any change in baffling when running with only one fan. It can beseen from FIG. 8 that diffuser element 14 and primary fan 8 may besimilarly removed without affecting the direction of the combinedairflow 22. All of these operations can be completed without shuttingdown the system contained in cabinet 4.

Referring back to the scenario at hand, a new secondary fan 16 may beset in place in sliding drawer 2, and the internal power and controlcable 44 may be re-connected to internal power and control receptacle46. Controller 40 may be configured to recognize that secondary fan 16has been replaced, and that it is operational, and to adjust the speedof primary fan 8 and secondary fan 16 accordingly. Sliding drawer 2 canthen be pushed back into cabinet 4 such that finger guard 6 and controlpanel 30 are flush with the outside of cabinet 4. Indicator lights 32may then be monitored by the operator for further problems. Indicatorlights 32 and controller 40 may also be interfaced with the system incabinet 4 to alert the operator through other means such as a remotesystem monitor.

Sliding drawer 2 may be configured to accommodate standard sized fansavailable from a variety of manufacturers, e.g. 120 mm, 92 mm, or 40 mmfans. These fans are readily available in a variety of thicknesses thatloosely correspond to a range of CFM ratings, i.e. the thicker fansgenerally have a higher CFM rating for a given fan diameter. It followsthat sliding drawer 2 may be configured to accept the thickest fan in aparticular size range, and that slimmer or lower capacity fans may beaccommodated by installing the fan in conjunction with a “shim” ringthat takes up the extra space and holds the fan securely in place. Thisapproach allows a standard size sliding drawer 2 to accommodate avariety of fan capacities, and also provides a convenient upgrade pathsince the shims may be removed or replaced with thinner shims to allowthe installation of higher capacity fans. This approach can be used toprovide additional cooling, when required, without replacing the entirecooling subsystem.

In some applications it may be necessary to provide a fixed baffle 48inside cabinet 4 to ensure that re-directed combined airflow 49 isappropriate for the application. This fixed baffle 48 will need tointerface with internal sleeve 3 to prevent air leakage, however it willremain fixed in the event of a fan failure.

FIG. 9 shows how two high performance series fan modules may be mountedin parallel for increased airflow. Parallel baffle 50 may be configuredto interface with top inner sleeve 3 a and bottom inner sleeve 3 b tocontain the output from both compact series fan assemblies, and producetotal combined airflow 54. Sealing cap 52 may be positioned between thetwo assemblies to improve the airflow and to prevent any leakage of airin this area. Sealing cap 52 may be configured with a cone shaped capthat protrudes downstream, or some other feature, to increase theefficiency of the airflow.

It is important to note that even though this is a parallelconfiguration of series fan assemblies, it does not require any of thespecialized baffling normally associated with this type of installation.This is because each one of the high performance series fans withdiffuser element assemblies is independently fault tolerant, andprevents the back flow of air in the event of a fan failure. In otherwords, each series fan assembly will always contribute to total combinedairflow 54, and will not allow a portion of combined airflow 54 to leakback out to the ambient air around cabinet 4, even in the event of asingle fan failure.

The parallel configuration of high performance series fan modules alsoprovides more flexibility in the event of a fan failure. In this case acontroller may be configured to speed up three additional fans, ratherthan just one in a non-parallel installation, to maintain a constanttotal combined airflow 54. It follows that parallel configurations withmore than two high performance series fans with diffuser elementassemblies will have an even greater ability to respond to a single fanfailure.

FIG. 10 shows a high performance series fan module configured with asupplementary air inlet and outlet to improve airflow in the event of afan failure.

Under normal operation, air inlet baffle 70 and air outlet baffle 72will direct the output from primary fan 8 and diffuser element 14through secondary fan 16 to form combined airflow 22, as previouslydescribed. Combined airflow 22 is further directed through air funnel 74which may have an opening size that approximates the opening size of thefans.

In the event of a primary fan 8 failure, air inlet baffle 70 may bemoved to position 70 a to reduce the input impedance seen by, andtherefore increase the flow of air into, secondary fan 16. Outlet baffle72 may remain in place to ensure that no air leaks from the output sideto the input side of secondary fan 16. Combined airflow 22 will becomprised solely of the output from secondary fan 16, part of which willflow through the defective primary fan 8 and another part of which willflow through the open inlet baffle 70 a.

Conversely, in the event of an secondary fan 16 failure, air outletbaffle 72 may be moved to position 72 a to reduce the output impedanceseen by, and therefore increase the flow of air out of, primary fan 8.In this case inlet baffle 70 will remain in place to ensure that no airleaks from the output side to the input side of primary fan 8. Combinedairflow 22 will be comprised solely of the output from primary fan 8,part of which will flow through the defective secondary fan 16 andanother part of which will flow through the open outlet baffle 72 a.

Inlet baffle 70 and outlet baffle 72 may be configured to operateautomatically, based on pressure differentials, or to be controlled bycontroller 40 (reference FIG. 8). In the former case a higher relativepressure between primary fan 8 and secondary fan 16 would cause outletbaffle 72 to move to position 72 a, and a lower relative pressurebetween the same fans would cause inlet baffle 70 to over to position 70a. In the latter case controller 40 may be used to control the positionof the baffles in response to a failing or defective fan. In all casesthe action taken serves to relieve the pressure differential and improvethe flow of air through the configuration. However the use of thecontroller provides greater flexibility and does allow for certain loadsharing scenarios between the two fans that might cause temporarypressure differentials between the fans that might otherwise beinterpreted as a defective fan situation.

It is important to note that air inlet baffle 70 and air outlet baffle72 may be configured, in conjunction with air funnel 74 and controller40 (reference FIG. 8), such that the direction and rate of combinedairflow 22 will remain constant even in the event of a fan failure.

This precludes the requirement for any further baffle changes withincabinet 4 in the event of a fan failure, meaning that the configurationmay still be supplied as a standalone module that provides faulttolerant cooling.

It is also important to note that the use of air inlet baffle 70 and airoutlet baffle 72 still allows for the replacement of a defective fan orfilter element/diffuser while the system is running. This is because airinlet baffle 70 and air outlet baffle 72 have been configured to notinterfere with the normal removal and replacement of the fan and filterelement/diffuser element while sliding drawer 2 is in the “out” positionas previously described.

Primary fan 8 and secondary fan 16 may both be mounted with axisparallel to combined airflow 22 as shown in FIG. 10. Alternatively,primary fan 8 and secondary fan 16 may both be mounted at a slight angleto the desired combined airflow 22, and not necessarily in a coaxialfashion, in order to improve the smooth flow of air between primary fan8 and secondary fan 16. In this case inner sleeve 3 and air funnel maybe adaptively re-configured to ensure that combined airflow 22 flows inthe desired direction.

FIG. 11 provides a connection diagram for high performance series fancontroller 40. Controller 40 may be configured to receive its primaryinput from cooled component(s) 62, upon which the output of highperformance cooling fan module 1, i.e. combined airflow 22, impinges.This primary input may be comprised of information such as thetemperature of cooled component(s) 62, the rate of airflow around cooledcomponent(s) 62, and the current and/or anticipated workload on cooledcomponent(s) 62. Information regarding the anticipated workload oncooled component(s) 62 would allow controller 40 to proactively respondto a corresponding change in heat dissipation requirements by changingthe speed of primary fan 8 and/or secondary fan 16.

Controller 40 may also be configured to receive input from airflowsensor 60. Airflow sensor 60 provides information regarding the rate ofcombined airflow 22, and this information may be used by controller 40to test for appropriate responses to changes in input to primary fan 8and/or secondary fan 16. A non-appropriate response to such an input maybe used by controller 40 to determine that there may be a fault withdiffuser element 14 or one of the fans. For example, controller 40 maydetermine that combined airflow 22 cannot be maintained above athreshold level and may deduce that (1) this problem may be caused by aseriously clogged diffuser element 14, especially if it has a secondaryfunction as a filter, or, in the worst case, that (2) both fans may havefailed or are failing simultaneously. The user would be alerted to takeimmediate action in either case, and a graceful shutdown procedure couldbe initiated if either situation persists for an unacceptable period oftime.

Controller 40 may also be configured to receive input from positionsensors 64, which inform controller 40 regarding the correct installedposition of primary fan 8, diffuser element 14, and secondary fan 16. Inthe case of the fans, this information may be combined with input fromcombined control and monitor wires 66 to determine that the fans areinstalled correctly and operating efficiently. The combined control andmonitor wires may be used to supply a control voltage to the fans,monitor current draw, and in some cases monitor other information suchas rpm, output temperature, or output flow rate.

Position sensors 64 may further contain a physical feature thatprecludes the incorrect installation of primary fan 8 and secondary fan16, i.e. prevents an accidental installation that would cause air toflow in the wrong direction. Such an incorrect installation could causeimmediate damage to the components being cooled.

The information provided by combined monitor and control wires 66 may beused by controller 40 as leading indicators of potential fan failure. Asan example, a drop in rpm for a given voltage input may indicate that abearing is failing. Controller 40 may initially respond by increasingthe voltage input to that fan, and alerting the user to the problem.Controller 40 may ultimately respond by shutting down the defective fanand changing the load over to the alternative fan if the problempersists. Most importantly, the information allows the controller tomake proactive responses to an impending problem before cooledcomponent(s) 62 becomes overheated.

Controller 40 may communicate with the user through control panel 30,containing indicator lights 32 a, 32 b, and 32 c, which may be used toindicate the status of primary fan 8, diffuser element 14, and secondaryfan 16 respectively. Any commonly understood indicator algorithm may beused, for example green meaning normal operation, yellow meaning that acomponent should be replaced due to sub-optimal performance or impendingfailure, and red or flashing red used to indicate that a component hasfailed. Note that a failed fan does not mean that high performancecooling fan module 1 is not operating; it simply means that the systemis only running with one fan and has no ability to respond to a furtherfan failure. Therefore the failed component must be replaced immediatelyto avoid potential problems.

As an example, controller 40 may be used to monitor the amount of timethat diffuser element 14 is in use, and to activate the appropriateindicator light 32 should the “in use” time exceed a recommendedmaximum. This will alert the operator to replace diffuser element 14.The appropriate position sensor 64 in may be used to automatically resetthe “in use” timer back to zero. This algorithm would be particularlyuseful in applications where diffuser element 14 is configured as acombined filter/diffuser element.

Controller 40 may also communicate with the user through a secondredundant set of internal indicator lights 33 (reference FIG. 8). Theselights may be more visible to the user or service technician when thefans are being replaced, and therefore they will serve as a safeguard toprevent the accidental removal of a correctly operating fan. Such amistake would leave only the defective fan in place, potentially causingimmediate damage to cooled component(s) 62. Controller 40 may use anaudible emergency signal to instantly warn the user of such a dangeroussituation.

FIG. 12 presents a control algorithm for a high performance series fancontroller, in flow chart format.

The fundamental purpose of the controller is to keep cooled component(s)62 (reference FIG. 11) within a defined control temperature range,despite changes on workload that might affect the heat dissipated bycooled component(s) 62. Therefore the first task in each control cycleis to check for anticipated changes in workload as outlined in firstdecision triangle 80. This information may come from the operatingsystem associated with cooled component(s) 62. An increase in workloadwould cause the controller to increase the output CFM control point, anda decrease in workload would cause the controller to decrease the outputCFM control point, perhaps after some delay period, as indicated byfirst control box 86. The controller would proceed directly to seconddecision triangle 82 should there be no anticipated changes in workload.

At second decision triangle 82 the controller will check to ensure thatcooled component(s) 62 (reference FIG. 11) is operating within itsdefined control temperature range. Should this not be the case, then thecontroller will adjust the output CFM control point to raise or lowerthe temperature of cooled component(s) 62 as required. However undernormal operation, when no adjustment is required, the controller willproceed directly to third decision triangle 84.

At third decision triangle 84, the controller checks to ensure that theoutput CFM, i.e. combined airflow 22 (reference FIG. 11), is at theoutput CFM control point. Should there be a discrepancy that liesoutside of the acceptable control range, then the controller willimmediately investigate to determine the cause of the problem. As anexample, secondary fan 16 (reference FIG. 11) may have suffered a dropin rpm given the same input parameters, a possible leading indicator ofimpending fan failure. The controller would then proceed to takecorrective action by adjusting the inputs to secondary fan 16 andnotifying the user through indicator lights 32 (reference FIG. 11).

Under normal circumstances the output of the high performance coolingfan module will be at the required constant output CFM control point andno corrective action will be required. In this case the controller loopsback to first decision triangle 80 to repeat the above control cycleonce again.

While operating normally, the controller may actually change the speedof both fans slightly on a regular timed basis. These subtle changes inrpm will prevent any lasting beat frequencies that might occur if thefans are left running at a constant rpm for any length of time.

Interrupts may be used at any time to alert the controller regarding asituation that requires immediate attention. Examples may include alocked rotor (“0” rpm with a full normal input) or perhaps a dislodgedfan. In these cases the controller must take immediate action topreserve a constant CFM output, thus keeping the cooled component(s) atthe required operating temperature.

FIG. 13 provides a perspective view of high performance series fan sink100. Primary fan 8 and secondary fan 16 are configured in series to drawinlet airflow 108 into high performance series fan module 106, and pushit into heat sink 102 where it divides into right outlet airflow 110 andleft outlet airflow 112. Primary fan 8 and secondary fan 16 may beobliquely mounted on heat sink 102 at a variety of angles such thediagonal of the fans substantially covers the width of heat sink 102 andprovides airflow through substantially all of the channels within heatsink 102. Air is retained within the confines of heat sink 102, suchthat it flows through and only exits at the open ends of heat sink 102,by baffle 104.

Baffle 104 may be configured to hold high performance series fan module106 at a distance above heat sink 102, while preventing the leakage ofair at the interface between baffle 104 and high performance series fanmodule 106, to improve the dispersion of air throughout heat sink 102.Further, baffle 104 may be configured to expand the opening of highperformance series fan module 106 such that covers substantially all ofthe width of heat sink 102, allowing smaller series fan modules 106 tobe used effectively with larger heat sinks 102.

Inlet airflow 108 is drawn through finger guard 122, into primary fan 8,through diffuser element 14, into secondary fan 16, and then pushedthrough heat sink 102 and exhausted as right outlet airflow 110 and leftoutlet airflow 112. Alternatively, the direction of airflow may bereversed such that right outlet airflow 110 and left outlet airflow 112become the inlet airflows, and the air is exhausted through finger guard122 at inlet airflow 108, which becomes the exhaust. However the formerconfiguration, as illustrated in FIG. 13, provides for an impingementair flow on heat sink 102, and this can be directed at the area ofmaximum heat flux on heat sink 102 for enhanced cooling efficiency.

Control module 120 controls the operation of high performance series fansink 100. Primary fan indicator light 122 and secondary fan indicatorlight 124 indicate the operating status of primary fan 8 and secondaryfan 16 respectively. Control module 120 may be configured to sense thefailure of primary fan 8 or secondary fan 16 and increase the power tosecondary fan 16 or primary fan 8, respectively, to maintain arelatively constant right outlet airflow 1 10 and left outlet airflow112 during a single fan failure. Further, control module 120 may beconfigured to be responsive to a range of different backpressures toprovide a relatively constant right outlet airflow 110 and left outletairflow 1 12 over a range of operating conditions, or for a variety ofheat sinks 102.

FIG. 14 provides a section view of high performance series fan sink 100.High performance series fan module 106 contains primary fan 8, diffuserelement 14, and secondary fan 16. High performance series fan module 106may be configured as a module that contains all of these components andholds them at the appropriate location, or alternatively as astandardized sub-assembly that only contains diffuser element 14 and isadapted to be bolted or otherwise fastened between two industry standardfans of similar geometry, e.g. two 120 mm or 40 mm fans.

Primary fan 8 is separated from diffuser element 14 by a first distance,and diffuser element 14 is further separated from secondary fan 16 by asecond distance. The purpose of the first distance between primary fan 8and diffuser element 14 is to reduce the swirl component of the airflowexiting from primary fan 8 through natural swirl decay, with a longerchannel generally resulting in an increased level of natural swirldecay. The first distance may be reduced by configuring the internalgeometry of the airflow channel to increase the rate of natural swirldecay, e.g. by using a square or octagonal internal cross section and/orby incorporating ridges, spines, or other surface features along theinterior walls of the airflow channel, thereby reducing the overalllength of high performance series fan module 106. The first distance maybe further reduced by selecting a primary fan 8 having an integratedstator on the outlet side, thereby providing some level of swirl decaybefore the airflow leaves primary fan 8.

The purpose of diffuser element 14 is to complement the natural swirldecay accomplished within the first distance, i.e. between primary fan 8and diffuser element 14, by further reducing the swirl component of theairflow before it enters secondary fan 16. This will increase theefficiency of secondary fan 16.

The purpose of the second distance between diffuser element 14 andsecondary fan 16 is to reduce the acoustical noise produced by highperformance series fan module 106. The small gap between the twocomponents also provides sufficient space to mount a pressure sensor,and this signal may be compared to the signal produced by anotherpressure sensor located on the upstream side of diffuser element 14 toprovide an indication of flow rate through high performance series fanmodule 106.

Thermal load 130 may be in thermal communication with the bottom of heatsink 102, and may be optimally positioned such that area of highest heatflux (i.e. the hottest portion of heat sink 102) is immediately belowthe impinging airflow. Heat may then be removed through forcedconvection as the air flows through heat sink 102 and exits as rightoutlet airflow 110 and left outlet airflow 112, as previously described.Control 1o module 120 may be configured to maintain a constanttemperature of thermal load 130, a constant right outlet airflow 110 andleft outlet airflow 112, or some combination of these and/or othercontrol parameters.

FIG. 15 illustrates high performance series fan sink 100 as primary fan8 is being replaced. A defective primary fan 8 may be removed whilethermal load 130 (reference FIG. 14) remains active since control module120 may be configured to increase the power applied to secondary fan 16during the primary fan 8 outage, and until primary fan 8 has beenreplaced, in order to maintain a relatively constant right outletairflow 110 and left outlet airflow 112 (reference FIG. 14). Controlmodule 120 may also be configured to detect the re-insertion of a newprimary fan 8, and may then re-apply power to both fans in a controlledfashion to optimize the performance of high performance series fanmodule 106, as previously described.

FIG. 16 illustrates high performance series fan sink 100 with secondaryfan 16 being replaced. A defective secondary fan 16 may be removed whilethermal load 130 (reference FIG. 14) remains active since control module120 will increase the power applied to primary fan 8 during thesecondary fan 16 outage, and until secondary fan 16 has been replaced,in order to maintain a relatively constant right outlet airflow 110 andleft outlet airflow 112 (reference FIG. 14). Control module 120 may alsobe configured to detect the re-insertion of a new secondary fan 18, andmay then re-apply power to both fans in a controlled fashion to optimizethe performance of high performance series fan module 106, as previouslydescribed.

FIG. 17 provides a perspective view of high performance series fan tray200, which may be configured with a single row of high performanceseries fan modules, as shown, or multiple rows of high performanceseries fan modules. Further, a single row of high performance series fanmodules may be configured as a partial fan tray that may be mounted fromthe front of a rack system, and possibility combined with a similar fantray mounted from the back of the same system to provide flexible andexpandable cooling solutions. Further, high performance series fan trays200 may be may be mounted horizontally to produce a vertical airflow, orvertically to produce a horizontal airflow. Finally, one or more highperformance series fan modules may be added to an existing fan tray,using a traditional array of single axial fans in parallel, to increaseperformance and add a measure of fault tolerance to an existinginstallation.

Each high performance series fan module within high performance seriesfan tray 200 may be configured independently. For example, one modulemay be configured with a duct to provide direct cooling for one or morecomponents within the system, and another module may be configured toactively exhaust air from the same or different component(s). Othermodules may be configured to provide a more general flow of air withinthe system.

The high performance series fan tray 200 depicted in FIG. 17 includesthree high performance series fan modules, 106 a, 106 b, and 106 c, thatdraw inlet airflows 108 a, 108 b, and 108 c, respectively, to produceoutlet airflows 110 a, 110 b, and 110 c, respectively. Control module120 may be configured to monitor and control high performance series fanmodules 106 a, 106 b, and 106 c, and outlet airflows 110 a, 110 b, and110 c

FIG. 18 provides a second perspective view of high performance seriesfan tray 200, showing further details of high performance series fanmodule 106 a (reference FIG. 17), which contains primary fan 8, diffuserelement 14, and secondary fan 16, and operates as previously described.High performance series fan module 106 a further contains primary fanindicator light 122 a and secondary fan indicator light 124 a.

It may be seen from FIG. 18 that control module 120 may contain CubicFeet per Minute (CFM) or temperature display 126, increase incrementbutton 130, decrease increment button 128, and power switch 132. TheCFM, temperature, or other set point may be increased or decreased bypressing increase increment button 130 or decrease increment button 128,respectively, causing control module 120 to adjust the power applied tohigh performance series fan modules 106 a, 106 b, and 106 c (referenceFIG. 17) accordingly. CFM or temperature display 126 may then be used tomonitor the changing parameter as it moves towards, and then reaches,the new set point.

FIG. 19 illustrates high performance series fan tray 200 with primaryfan 8 c being replaced. Primary fan 8 c may be removed while the thermalload within the cabinet or system being cooled remains active sincecontrol module 120 will increase the power applied to secondary fan 16c, and high performance series fan modules 106 a and 106 b (referenceFIG. 17), during the primary fan 8 c outage, and until primary fan 8 chas been replaced, in order to maintain a relatively constant combinedoutlet airflow, comprised of output airflows 110 a, 110 b, and 110 c(reference FIG. 17). Control module 120 may also be configured to detectthe re-insertion of a new primary fan 8 c, and then re-apply power tohigh performance series fan modules 106 a, 106 b, and 106 c in abalanced fashion in order to optimize the performance of highperformance series fan tray 100, as previously described.

FIG. 20 illustrates high performance series fan tray 200 with secondaryfan 16 c being replaced. Secondary fan 16 c may be removed while thethermal load within the cabinet or system being cooled remains activesince control module 120 will increase the power applied to primary fan8 c, and high performance series fan modules 106 a and 106 b (referenceFIG. 17), during the secondary fan 16 c outage, and until secondary fan16 c has been replaced, in order to maintain a relatively constantcombined outlet airflow, comprised of output airflows 110 a, 110 b, and110 c (reference FIG. 17). Control module 120 may also be configured todetect the re-insertion of a new secondary fan 16 c, and then tore-apply power to high performance series fan modules 106 a, 106 b, and106 c in a balanced fashion in order to optimize the performance of highperformance series fan tray 100, as previously described.

FIG. 21 illustrates control module 120 operating in fan failure mode.Control module 120 is in communication with, and controls the powerdelivered to, primary fan modules 8 a, 8 b, and 8 c, and secondary fanmodules 16 a, 16 b, and 16 c (reference FIG. 18, 19, 20), and theirrespective indicator lights. Control module 120 may be configured tosense that secondary fan module 16 b has failed, and to illuminatesecondary fan module indicator light 124 b accordingly. Controllermodule 120 may then adjust the power applied to cooling fan modules 106a, 106 b, and 106 c such that adjusted inlet airflows 138 a and 138 care greater than normal inlet airflow 108 (shown here for referenceonly), and adjusted inlet airflow 138 b, solely generated by primary fanmodule 114 b, is as close to normal inlet air 108 as possible. Inletflows 138 a, 138 b, and 138 c may be adjusted in this manner such thatthe combined outlet airflow will be substantially equal to the sum ofcombined normal outlet airflows 110 a, 110 b, and 110 c, and the thermalload within the system or cabinet being cooled will experience the samedegree of forced convection cooling as with normal operation. Controlmodule 120 may be configured to compensate for multiple fan modulefailures in a similar manner, however at some point the remaining fansmay not be able to generate the full replacement airflow during theoutage situation. Further, control module 120 may be configured tore-adjust power delivered to the cooling fan modules to normal levelsonce the defective fan(s) have been replaced, and turn off the indicatorlights accordingly.

FIG. 22 illustrates a method for monitoring the airflow through highperformance series fan module 106 using first pressure sensor 142 andsecond pressure sensor 144. Control module 120 may be in communicationwith both sensors, and may be configured to monitor the output from bothsensors to determine the differential pressure between first pressuresensor 142 and second pressure sensor 144, as caused by the flow of airthrough diffuser element 14. Control module 120 may then use thedifferential pressure information to determine the rate of flow of airthrough diffuser element 14, and may further use the flow rateinformation as a feedback signal for an internal flow rate controlalgorithm. The power applied to primary fan 8 and secondary fan module16 may be adjusted by control module 120 to compensate for any detecteddifference between the measured flow rate and the flow set point forhigh performance series fan module 106. A power adjustment that does notgenerate the predicted response, or does not generate a response thatfalls within normal guidelines, may indicate to the controller thatprimary fan 8 or secondary fan 16 is failing or has failed. Controlmodule 120 may complete further tests, in like manner, to determinewhich fan has a problem, to determine the extent of that problem, and todetermine an appropriate response.

FIG. 22 also illustrates swirl gap 140 between primary fan 8 anddiffuser element 14. The swirl component of the flow produced by primaryfan 8 will decay at an initial rate, and then decay at an everdecreasing rate as the distance from primary fan 8 increases. Swirl gap140 allows sufficient space for some decay of swirl prior to diffuserelement 14. This increases the effectiveness of diffuser element 14since the swirl component at the inlet side of diffuser element 14 willhave been reduced by some amount, and the net swirl decay caused byswirl gap 140 combined with diffuser element 14 will be greater thanthat caused by a diffuser element 14 placed immediately downstream fromprimary fan 8. The location and physical characteristics of diffuser 14may be configured such that the swirl and other flow parameters meet orexceed the design specifications for secondary fan 16 as the flow enterssecondary fan 16.

A small gap may be introduced between diffuser element 14 and secondaryfan module 118 to reduce the acoustical noise produced high performanceseries fan module 106, and to allows sufficient space for secondpressure sensor 144. This gap may be eliminated if second pressuresensor 144 is placed within diffuser element 14 116, at some distancefrom first pressure sensor 142, and if acoustic management is not anoverriding design consideration.

Although diffuser element 14 has a very positive effect on theefficiency and performance of high performance series fan module 106, aspreviously described, it does introduce a small flow restriction and acorresponding pressure drop. Although this is acceptable during normaloperation, it does limit the maximum achievable flow rate when only oneof primary fan 8 or secondary fan module 16 is operational. Therefore insome applications diffuser element 14 may be configured to slide out ofthe way, swing out of the way, or otherwise be partially or completelyremoved from the flow in order to maximize the achievable flow rateduring an outage situation.

Accordingly, diffuser element 14 may be configured to be removable fromthe flow by splitting it in the middle, and allowing each half to swingtowards primary fan module 8. The right half of diffuser element 14 andthe left half of diffuser element 14 may be configured to swing alongthe right and left sides of high performance series fan module 106,respectively, and lie along the sides of the airflow channel in the areanormally defined as swirl gap 140 during a fan outage situation. Thesides of swirl gap 140 may be configured to accommodate the right andleft sides of diffuser element, so positioned, such that they present aminimum restriction to the flow. Control module 120 may be configured torelease the right and left sides of diffuser element 14 during a fanoutage, such that they must be manually returned to normal position whenthe defective fan has been replaced, and held there with a retainingmechanism controlled by control module 120, or to move the right andleft sides of diffuser element 14 in a controlled fashion both duringthe outage and after it has been resolved.

FIG. 23 provides a perspective view of an alternatively configured highperformance series fan tray with high performance series fan modules 106a and 106 b mounted obliquely to provide a relatively even airflow overthe maximum width possible with only two high performance cooling fanmodules. Further, the primary and secondary cooling fans located withinhigh performance series fan modules 106 a and 106 b, so mounted, may beconveniently removed by sliding them in the direction defined by removalarrows 156 and 154, respectively. Multiple high performance series fanmodules may be configured obliquely, in this manner, and at variousangles, to provide a relatively even airflow over a maximum possiblewidth with the fewest possible number of high performance series fanmodules. Further, this configuration offers fault tolerance with thefewest possible number of high performance series fan modules.

The present invention may be embodied in other specific forms withoutdeparting from the spirit or essential characteristics thereof. Certainadaptations and modifications of the invention will be obvious to thoseskilled in the art. Therefore, the above-discussed embodiments areconsidered to be illustrative and not restrictive, the scope of theinvention being indicated by the appended claims rather than theforegoing description, and all changes which come within the meaning andrange of equivalency of the claims are therefore intended to be embracedtherein.

1. A series fan assembly comprising: a) A primary fan; b) A secondaryfan; in series with said primary fan; c) A flow modification element;configured to reduce swirl and mounted between said primary fan and saidsecondary fan; d) A connecting sleeve, wherein said connecting sleevedirects the output of said primary fan through said flow modificationelement and into said secondary fan.
 2. A series fan assembly as claimedin claim 1, wherein said connecting sleeve further directs the combinedoutput of said primary fan and said secondary fan into an enclosurecontaining components to be cooled;
 3. A series fan assembly as claimedin claim 1, wherein said series fan assembly is configured to maintainthe combined output of said primary fan and said secondary fan above aminimum level at all times, in the event of the failure of said primaryfan or said secondary fan.
 4. A series fan assembly as claimed in claim1 further comprising more than two fans connected in series, eachseparated by a distance and an appropriate said flow modificationelement.
 5. A series fan assembly as claimed in claim 1 wherein saidflow modification element is a filter.
 6. A series fan assembly asclaimed in claim 1 wherein said flow modification element is a heatexchanger.
 7. A series fan assembly as claimed in claim 1 wherein saidflow modification element is an electromagnetic shield.
 8. A series fanassembly as claimed in claim 1 wherein said secondary fan is mounted adistance from said flow control element to reduce acoustic noise.
 9. Aseries fan assembly as claimed in claim 1 wherein said flow modificationelement is comprised of a series of vanes or tubes configured coaxiallywith said primary fan and said secondary fan.
 10. A series fan assemblyas claimed in claim 1 wherein said flow modification element iscomprised of a series of vanes or tubes configured to create aspiralling laminar flow of air over the fixed blades of said primary fanor said secondary fan when defective.
 11. A series fan assembly asclaimed in claim 1 wherein said flow modification element is comprisedof a series of tubes with an air funnel at each entry point, said airfunnels opening towards and skewed towards the source of the airflow asit comes off the blades of said primary fan.
 12. A series fan assemblyas claimed in claim 1 wherein the fan blades of said primary fan and thefan blades of said secondary fan may be configured with adjustable pitchto return to a low airflow impedance position when locked.
 13. A seriesfan assembly as claimed in claim 1 wherein said primary fan and saidsecondary fan both normally operate at less than full rotating speed.14. A series fan assembly as claimed in claim 1 wherein the rotatingspeed of said primary fan or said secondary fan may be increased tocompensate for the failure of another fan.
 15. A series fan assembly asclaimed in claim 1 wherein two or more such series fan assemblies may bemounted in parallel to provide greater performance and fault tolerance.16. A series fan assembly as claimed in claim 1 further comprising anindicator means to alert an operator regarding the location and statusof a faulty component.
 17. A series fan assembly as claimed in claim 1further comprising a physical means to prevent the accidental reverseinstallation of said primary fan, said flow modification element, orsaid secondary fan.
 18. A series fan assembly as claimed in claim 1wherein said primary fan and said secondary fan may rotate in the sameor different directions.
 19. A series fan assembly as claimed in claim 1wherein said primary fan and said secondary fan may have the same ordifferent capacity ratings.
 20. A series fan assembly as claimed inclaim 1 wherein said primary fan and/or said secondary fan may have anintegrated stator on the outlet side.
 21. A series fan assembly asclaimed in claim 1 wherein the direction of flow of said combined outputremains consistent in the event of a failure of said primary fan or thefailure of said secondary fan.
 22. A series fan assembly as claimed inclaim 1 further comprising a means to attach said connecting sleeve tosaid enclosure.
 23. A series fan assembly as claimed in claim 1 furthercomprising sensors attached to said primary fan and said secondary fan,and capable of predicting the impending failure of said primary and saidsecondary fan.
 24. A series fan assembly as claimed in claim 1 whereinsaid connecting sleeve may be configured to accommodate a variety ofstandard size fans.
 25. A series fan assembly as claimed in claim 1wherein said connecting sleeve may be configured with octagonal cornersor other internal features capable of flow modification.
 26. A seriesfan assembly as claimed in claim 1 wherein said connecting sleeve andsaid flow modification element may be configured as a independent moduleto be later attached to a variety of standard size fans.
 27. A seriesfan assembly as claimed in claim 1 further comprising shims to allow theinstallation of less than maximum capacity standard sized fans, saidshims being installed with said primary fan or said secondary fan tohold it securely in place; wherein said shims may be removed at any timeto allow said primary fan or said secondary fan to be upgraded.
 28. Aseries fan assembly as claimed in claim 1 wherein said primary fan andsaid secondary fan form an integral part of said connecting sleeve. 29.A series fan assembly as claimed in claim 1 wherein said connectingsleeve is adapted to direct a flow of air into a heat sink.
 30. A seriesfan assembly as claimed in claim 1 wherein said connecting sleeve isadapted to mount obliquely on the cooling fin surface of a heat sink andto direct an impingement flow of air into said heat sink.
 31. A seriesfan assembly as claimed in claim 1 further comprising a controller,wherein said controller is configured to maintain said combined outputabove a minimum control level at all times, in the event of the failureof said primary fan or said secondary fan.
 32. A series fan assemblywith baffles comprising: a) A primary fan; b) A secondary fan; in serieswith said primary fan; S c) A flow modification element; configured toreduce swirl and mounted between said primary fan and said secondaryfan; d) An air inlet baffle configured to allow the free flow of airpast said primary fan in response to a failed said primary fan; e) Anair outlet baffle configured to allow the free flow of air past saidsecondary fan in response to a failed said secondary fan f) At least onesensor monitoring the status of each of said primary fan and saidsecondary fan; g) A power source; h) A controller in communication withsaid sensors, said power source, said primary fan, and said secondaryfan; i) A connecting sleeve, wherein said connecting sleeve directs theoutput of said primary fan through said flow modification element andinto said secondary fan; said connecting sleeve further configured toaccommodate said air inlet baffle and said air outlet baffle.
 33. Aseries fan assembly with baffles as claimed in claim 32 wherein saidconnecting sleeve further directs the combined output of said primaryfan and said secondary fan into an enclosure containing components to becooled.
 34. A series fan assembly with baffles as claimed in claim 32wherein said controller is configured to maintain the combined output ofsaid primary fan and said secondary above a minimum control level at alltimes, in the event of the failure of said primary fan or said secondaryfan.
 35. A series fan assembly with baffles as claimed in claim 32further comprising more than two fans connected in series, eachseparated by an appropriate said flow modification element.
 36. A seriesfan assembly with baffles as claimed in claim 32 wherein said flowmodification element is a filter.
 37. A series fan assembly with bafflesas claimed in claim 32 wherein said flow modification element is a heatexchanger.
 38. A series fan assembly with baffles as claimed in claim 32wherein said secondary fan is mounted a distance from said flow controlelement to reduce acoustic noise.
 39. A series fan assembly with bafflesas claimed in claim 32 wherein said flow modification element iscomprised of a series of vanes or tubes configured coaxially with saidprimary fan and said secondary fan.
 40. A series fan assembly withbaffles as claimed in claim 32 wherein said flow modification element iscomprised of a series of vanes or tubes configured to create aspiralling laminar flow of air over the fixed blades of said primary fanor said secondary fan when defective.
 41. A series fan assembly withbaffles as claimed in claim 32 wherein said flow modification element iscomprised of a series of tubes with an air funnel at each entry point,said air funnels opening towards and skewed towards the source of theairflow as it comes off the blades of said primary fan.
 42. A series fanassembly with baffles as claimed in claim 32 wherein the fan blades ofsaid primary fan and the fan blades of said secondary fan may beconfigured with adjustable pitch to return to a low airflow impedanceposition when locked.
 43. A series fan assembly with baffles as claimedin claim 32 wherein said primary fan and said secondary fan bothnormally operate at less than full rotating speed.
 44. A series fanassembly with baffles as claimed in claim 32 wherein the rotating speedof said primary fan or said secondary fan may be increased to compensatefor the failure of another fan.
 45. A series fan assembly with bafflesas claimed in claim 32 wherein two or more such series fans with bafflesmay be mounted in parallel to provide greater fault tolerance.
 46. Aseries fan assembly with baffles as claimed in claim 32 furthercomprising an indicator means to alert an operator regarding thelocation and status of a faulty component.
 47. A series fan assemblywith baffles as claimed in claim 32 further comprising a physical meansto prevent the accidental reverse installation of said primary fan, saidflow modification element, or said secondary fan.
 48. A series fanassembly with baffles as claimed in claim 32 wherein said primary fanand said secondary fan may rotate in the same or different directions.49. A series fan assembly with baffles as claimed in claim 32 whereinsaid primary fan and said secondary fan may have the same or differentcapacity ratings.
 50. A series fan assembly with baffles as claimed inclaim 32 wherein said primary fan and/or said secondary fan may have anintegrated stator on the outlet side.
 51. A series fan assembly withbaffles as claimed in claim 32 wherein the direction of flow of saidcombined output remains consistent in the event of a failure of saidprimary fan or the failure of said secondary fan.
 52. A series fanassembly with baffles as claimed in claim 32 further comprising a meansto attach said connecting sleeve to said enclosure.
 53. A series fanassembly with baffles as claimed in claim 32 further comprising sensorsattached to said primary fan and said secondary fan, and capable ofpredicting the impending failure of said primary and said secondary fan.54. A series fan assembly with baffles as claimed in claim 32 whereinsaid connecting sleeve may be configured to accommodate a variety ofstandard size fans.
 55. A series fan assembly with baffles as claimed inclaim 32 wherein said connecting sleeve may be configured with octagonalcorners or other internal features capable of flow modification.
 56. Aseries fan assembly with baffles as claimed in claim 32 wherein saidconnecting sleeve and said flow modification element may be configuredas a independent module to be later attached to a variety of standardsize fans.
 57. A series fan assembly with baffles as claimed in claim 32further comprising shims to allow the installation of less than maximumcapacity standard sized fans, said shims being installed with saidprimary fan or said secondary fan to hold it securely in place; whereinsaid shims may be removed at any time to allow said primary fan or saidsecondary fan to be upgraded.
 58. A series fan assembly with baffles asclaimed in claim 32 wherein said primary fan and said secondary fan forman integral part of said connecting sleeve.
 59. A series fan assemblywith baffles as claimed in claim 32 wherein said connecting sleeve isadapted to direct a flow of air into a heat sink.
 60. A series fanassembly with baffles as claimed in claim 32 wherein said connectingsleeve is adapted to mount obliquely on the cooling fin surface of aheat sink and to direct an impingement flow of air into said heat sink.61. A series fan assembly with baffles as claimed in claim 32 whereinsaid air inlet baffle and said air outlet baffle may be configured toautomatically respond to changes in relative air pressure.
 62. A seriesfan assembly with baffles as claimed in claim 32 wherein the position ofsaid air inlet baffle and said outlet baffle may be controlled by saidcontroller.
 63. A series fan assembly with baffles as claimed in claim32 wherein said primary fan and said secondary fan are configured withan offset yet parallel axis, wherein said axis is also parallel to saidcombined output.
 64. A series fan assembly with baffles as claimed inclaim 32 wherein said primary fan and said secondary fan may beconfigured at an angle to said parallel axis, and not necessarily incoaxial fashion.
 65. A series fan assembly with baffles as claimed inclaim 32 wherein said controller is in communication with the operatingsystem associated with the system contained in said enclosure, whereinsaid operating system may inform said controller of upcoming changes incooling requirements.
 66. A series fan assembly with baffles as claimedin claim 32 further comprising a temperature sensor in thermalcommunication with the component(s) being cooled, wherein saidtemperature sensor is also in communication with said controller, andwherein said controller may respond to changes the temperature of saidcomponent(s).
 67. A series fan drawer assembly comprising: a) A primaryfan; b) A secondary fan; in series with said primary fan; c) A flowmodification element; configured to reduce swirl and mounted betweensaid primary fan and said secondary fan; d) A connecting sleeve, e) Asliding drawer configured to detachably hold said primary cooling fan,said flow modification element, and said secondary cooling fan; saiddrawer further configured to slide into and out of said connectingsleeve; f) At least one sensor monitoring the status of each of saidprimary cooling fan and said secondary cooling fan; g) A power source h)A controller in communication with said sensors, said power source, saidsecondary fan, and said primary fan; wherein said connecting sleevedirects the output of said primary fan through said flow modificationelement and into said secondary fan.
 68. A series fan drawer assembly asclaimed in claim 67 wherein said connecting sleeve further directs thecombined output of said primary fan and said secondary fan into anenclosure containing components to be cooled;
 69. A series fan drawerassembly as claimed in claim 67 wherein said controller is configured tomaintain the combined output of said primary fan and said secondary fanabove a minimum control level at all times, in the event of the failureof said primary fan or said secondary fan.
 70. A series fan drawerassembly as claimed in claim 67 further comprising more than two fansconnected in series, each separated by an appropriate said flowmodification element.
 71. A series fan drawer assembly as claimed inclaim 67 wherein said flow modification element is a filter.
 72. Aseries fan drawer assembly as claimed in claim 67 wherein said flowmodification element is a heat exchanger.
 73. A series fan drawerassembly as claimed in claim 67 wherein said flow modification elementis an electromagnetic shield.
 74. A series fan drawer assembly asclaimed in claim 67 wherein said secondary fan is mounted a distancefrom said flow control element to reduce acoustic noise.
 75. A seriesfan drawer assembly as claimed in claim 67 wherein said flowmodification element is comprised of a series of vanes or tubesconfigured coaxially with said primary fan and said secondary fan.
 76. Aseries fan drawer assembly as claimed in claim 67 wherein said flowmodification element is comprised of a series of vanes or tubesconfigured to create a spiralling laminar flow of air over the fixedblades of said primary fan or said secondary fan when defective.
 77. Aseries fan drawer assembly as claimed in claim 67 wherein said flowmodification element is comprised of a series of tubes with an airfunnel at each entry point, said air funnels opening towards and skewedtowards the source of the airflow as it comes off the blades of saidprimary fan.
 78. A series fan drawer assembly as claimed in claim 67wherein the fan blades of said primary fan and the fan blades of saidsecondary fan may be configured with adjustable pitch to return to a lowairflow impedance position when locked.
 79. A series fan drawer assemblyas claimed in claim 67 wherein said primary fan and said secondary fanboth normally operate at less than full rotating speed.
 80. A series fandrawer assembly as claimed in claim 67 wherein the rotating speed ofsaid primary fan or said secondary fan may be increased to compensatefor the failure of another fan.
 81. A series fan drawer assembly asclaimed in claim 67 wherein two or more such series fan drawerassemblies may be mounted in parallel to provide greater faulttolerance.
 82. A series fan drawer assembly as claimed in claim 67further comprising an indicator means to alert an operator regarding thelocation and status of a faulty component.
 83. A series fan drawerassembly as claimed in claim 67 further comprising a physical means toprevent the accidental reverse installation of said primary fan, saidflow modification element, or said secondary fan.
 84. A high performanceseries fan drawer assembly as claimed in claim 53 wherein said primaryfan and said secondary fan may rotate in the same or differentdirections.
 85. A series fan drawer assembly as claimed in claim 67wherein said primary fan and said secondary fan may have the same ordifferent capacity ratings.
 86. A series fan drawer assembly as claimedin claim 67 wherein said primary fan and/or said secondary fan may havean integrated stator on the outlet side.
 87. A series fan drawerassembly as claimed in claim 67 wherein the direction of flow of saidcombined output remains consistent in the event of a failure of saidprimary fan or the failure of said secondary fan.
 88. A series fandrawer assembly as claimed in claim 67 further comprising a means toattach said connecting sleeve to said enclosure.
 89. A series fan drawerassembly as claimed in claim 67 further comprising sensors attached tosaid primary fan and said secondary fan, and capable of predicting theimpending failure of said primary and said secondary fan.
 90. A seriesfan drawer assembly as claimed in claim 67 wherein said connectingsleeve may be configured to accommodate a variety of standard size fans.91. A series fan drawer assembly as claimed in claim 67 wherein saidconnecting sleeve may be configured with octagonal corners or otherinternal features capable of flow modification.
 92. A series fan drawerassembly as claimed in claim 67 further comprising shims to allow theinstallation of less than maximum capacity standard sized fans, saidshims being installed with said primary fan or said secondary fan tohold it securely in place; wherein said shims may be removed at any timeto allow said primary fan or said secondary fan to be upgraded.
 93. Aseries fan drawer assembly as claimed in claim 67 wherein said primaryfan and said secondary fan form an integral part of said connectingsleeve.
 94. A series fan drawer assembly as claimed in claim 67 whereinsaid controller is in communication with the operating system associatedwith the system contained in said enclosure, wherein said operatingsystem may inform said controller of upcoming changes in coolingrequirements.
 95. A series fan drawer assembly as claimed in claim 67further comprising a temperature sensor in thermal communication withthe component(s) being cooled, wherein said temperature sensor is alsoin communication with said controller, and wherein said controller mayrespond to changes the temperature of said component(s).
 96. A seriesfan drawer assembly as claimed in claim 67 further comprising aredundant indictor means to confirm the identity of the faultycomponent, said redundant indicator means being visible when saidsliding drawer is pulled out from said enclosure.
 97. A high performanceseries fan drawer assembly as claimed in claim 53 wherein said slidingdrawer may be pulled out from said enclosure in a limited and controlledfashion while the system within said enclosure is still in operation.98. A series fan drawer assembly as claimed in claim 67 wherein saidprimary fan, said flow modification element, or said secondary fan maybe replaced while said drawer is pulled out from said enclosure andwhile the system within said enclosure is still in operation.
 99. Aseries fan drawer assembly as claimed in claim 67 wherein said slidingdrawer may be completely removed from said connecting sleeve and saidenclosure when required.
 100. A series fan drawer assembly as claimed inclaim 67 wherein said controller is in communication with the operatingsystem associated with the system contained in said enclosure, whereinsaid operating system may inform said controller of upcoming changes incooling requirements.
 101. A series fan drawer assembly as claimed inclaim 67 further comprising a temperature sensor in thermalcommunication with the component(s) being cooled, wherein saidtemperature sensor is also in communication with said controller, andwherein said controller may respond to changes the temperature of saidcomponent(s).